dojox.image.Magnifier is used to magnify an image that only makes an image larger. It cannot be used to show anything that dos not appear on the image. While a map maginifier will zoom the map in different zoom level, which may display some detail information that does not appear on the main map. So dojox.image.Magnifier cannot serve for the purpose of a map magnifier.
Unfortunately, ESRI JS API does not provide a magnifying dijit.
An idea is to create a movable div using dojo.dnd.Movable. Inside the movable div will be another map object whose zoom level will be the main map zoom level plus a number that designated by the magnifier. When drag and move the div, the magnifying map object will need to change its extent or center accordingly.